案例：jQuery案例     状态：可编辑再运行    进入竖版
 运行结果　
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<!DOCTYPE html>
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<html>
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<head>
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  <style>
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  div { width:70px; height:70px; background:#abc;
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        border:2px solid black; margin:10px; float:left; }
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  div.before { border-color: red; }
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  </style>
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  <script type="text/javascript" src="/js/jquery.js"></script>
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</head>
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​
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<body>
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  <div></div>
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  <div></div>
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  <div></div>
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  <div></div>
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<script>$("div:last").prevAll().addClass("before");</script>
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​
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</body>
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</html>
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​